Transmission and method of engagement

ABSTRACT

While a transmission range selector is in a Park position, a controller engages multiple transmission shift elements to place the transmission in a tie-up state. In the tie-up state, both the transmission output shaft and the turbine shaft are held against rotation by the shift elements. To transition from Park to Drive, the controller releases one of the shift elements to place the transmission in a 1st gear state. To transition from Park to Reverse, the controller releases a different one of the shift elements to place the transmission in a reverse state. The controller may wait until the driver releases the brake pedal to release the shift element.

TECHNICAL FIELD

The present disclosure relates to the field of controls of automatictransmissions. In particular, it relates to a method of controllingtransmission shift elements as a shift selector is moved among Park,Reverse, Neutral, and Drive positions.

BACKGROUND

Many vehicles utilize automatic transmissions to transmit power from aninternal combustion engine to the vehicle wheels. The automatictransmission adjusts the speed ratio between the engine and the wheelssuch that the engine can run at an efficient speed as the vehicle speedvaries. At low vehicle speed, the transmission causes the wheels torotate much slower than the engine while delivering a multiple of theengine torque to the wheels. At high speed, the transmission permits theengine to run at a relatively low speed even as the wheels rotaterelatively fast. In reverse, the transmission delivers torque in theopposite direction causing the vehicle to move backwards even as theengine continues to rotate in the same direction.

The driver controls the transmission mode by manipulating a rangeselector. Range selectors typically provide Park, Reverse, Neutral, andDrive modes. In Park mode, the vehicle is held stationary even in thepresence of substantial force. This is conventionally implemented byplacing the transmission gearbox in a neutral state and holding theoutput shaft stationary with a parking pawl. In Reverse, the vehiclemoves backward when the driver presses the accelerator pedal. InNeutral, pressing the accelerator pedal does not cause the vehicle tomove although the vehicle is allowed to move in response to other forcessuch as gravity. Finally, in Drive, the vehicle moves forward when thedriver presses the accelerator pedal. Reverse, Neutral, and Drive areconventionally implemented by placing the gearbox in a correspondingstate with the parking pawl released.

When a driver moves the range selector from Park to Reverse or Drive.The gearbox must transition from a neutral state to a reverse gear ratioor a 1st gear ratio, respectively. In many automatic transmissions, thisis accomplished by engaging one or more shift elements. Unfortunately,there may be a delay between the time the transmission begins engaging ashift element and the time at which the shift element reaches therequired torque capacity to establish the reverse gear ratio or 1st gearratio. The delay may be longer when the engine is running slowly. Duringthis interval, the transmission is in neutral even though the driver hasselected Reverse or Drive. If the driver presses the accelerator pedalduring this interval, the engine speed will increase without causing thevehicle to move. If the vehicle is on a hill, it will begin to roll downthe hill. Then, when the transmission finishes the engagement, thevehicle may lurch suddenly in the selected direction. If the driver isholding the brakes as the transmission finishes the engagement, thepowertrain will begin to exert force against the brakes and the vehicleoccupants may notice a jolt from the reaction torque at the transmissionmounts.

SUMMARY

An automatic transmission includes a plurality of shift elements. Afirst shift element is engaged in Park and in a forward launch mode andis disengaged in a reverse launch mode. A second shift element isengaged in Park and in the reverse launch mode and is disengaged in theforward launch mode. The transmission may include a controller thatreleases the first clutch to transition from Park to Reverse andreleases the second clutch to transition from Park to Drive. Someembodiments may include one or more additional shift elements that areengaged in Park, the forward launch mode, and the reverse launch mode.While in Park, these shift elements hold both a transmission outputshaft and a turbine shaft against rotation.

A method of operating a transmission includes engaging sufficient shiftelements to hold an output shaft and a turbine shaft against rotationand then transitioning from Park to a non-Park mode by disengaging aparking pawl and disengaging one of the shift elements. The non-Parkmode may be, for example, a Reverse mode, a Drive mode, or a Neutralmode. The disengagement of the parking pawl may precede thedisengagement of the shift element. The disengagement of the shiftelement may be delayed until the drive also released a brake pedal.Engagement of the shift elements may be sequenced to avoid temporarilyestablishing a power flow path from the turbine shaft to the outputshaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a first exemplary transmissionconfiguration.

FIG. 2 is a flow chart illustrating a method of engaging and releasingshift elements in response to movement of a shift selector and actuationof a brake pedal.

FIG. 3 is a flow chart illustrating a sequence of shift elementengagement for the shift elements of the transmission of FIG. 1.

FIG. 4 is a schematic representation of a second exemplary transmissionconfiguration.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

FIG. 1 illustrates a representative automatic transmission. Thetransmission is contained in a housing 12 that is fixed to vehiclestructure. An input shaft 14 is driven by the vehicle engine. An outputelement 16 drives vehicle wheels. A torque converter 18 has an impellerfixed to input shaft 14 and a turbine fixed to turbine shaft 20. Torqueconverter 18 transmits torque from input shaft 14 to turbine shaft 20while permitting turbine shaft 20 to rotate slower than input shaft 14.When turbine shaft 20 rotates substantially slower than input shaft 14,the torque converter stator is held against rotation by one way clutch22 such that the torque applied to turbine shaft 20 is a multiple of thetorque supplied at input shaft 14. Alternatively, torque converter 18could be replaced by a launch clutch.

Gear box 24 establishes a number of speed ratios between turbine shaft20 and output element 16. Specifically, gear box 24 has three planetarygear sets and five shift elements that establish six forward and onereverse speed ratio. Simple planetary gear sets 26, 28, and 30 each havea sun gear, a carrier, and a ring gear that rotate about a common axis.Each planetary gear set also includes a number of planet gears thatrotate with respect to the carrier and mesh with both the sun gear andthe ring gear. The carrier of gear set 26 is fixedly coupled to the ringgear of gear set 28. The carrier of gear set 28 is fixedly coupled thering gear of gear set 30. The ring gear of gear set 26 is fixedlycoupled to the carrier of gear set 30 and to output element 16. Finally,the sun gear of gear set 28 is fixedly coupled to turbine shaft 20.

The various speed ratios are established by engaging variouscombinations of shift elements. A shift element that selectively holds agear element against rotation may be called a brake whereas a shiftelement that selectively couples two rotating elements to one anothermay be called a clutch. Clutches 32 and 34 selectively couple turbineshaft 20 to the carrier and sun gear, respectively, of gear set 26.Brakes 36 and 38 selectively hold sun gears gear sets 26 and 30,respectively, against rotation. Brake 40 selectively holds the carrierof gear set 26 against rotation. Finally, one way clutch passively holdsthe carrier of gear set 26 against rotation in one direction whileallowing rotation in the opposite direction. Table 1 illustrates whichshift elements are engaged to establish each speed ratio.

TABLE 1 32 34 36 38 40/42 Ratio Step Reverse X X −3.00 71% 1st X X 4.202nd X X 2.70 1.56 3rd X X 1.80 1.50 4th X X 1.40 1.29 5th X X 1.00 1.406th X X 0.75 1.33

Shift elements 32-40 may be hydraulically actuated multi-plate wetfriction clutches or brakes. Engine driven pump 44 pressurizes hydraulicfluid. Controller 46 selectively routes the pressurized fluid to theshift elements to be engaged. This may be accomplished by controllingvalves in a valve body to alternately connect each shift element eitherto the high pressure supply or to a vent. In some embodiments, thecontroller may adjust the current to one of more variable forcesolenoids to control the pressure supplied to each clutch. When theengine is not running, the pump does not supply pressurized fluid so allof the shift elements are disengaged. When pressurized fluid is firstsupplied to a shift element, it moves a piston into a stroked position.Then, the fluid forces plates together so that the shift element cantransmit torque. The torque capacity is negligible until the pistonreaches the stroked position. When the pressure is relieved, the torquecapacity rapidly drops to near zero and then the piston continues tomove to a released (not stroked) position.

When the range selector is in the Park position, a parking pawl 48 isengaged which holds output element 16 against rotation with respect tothe housing 12. The parking pawl 48 is designed to remain engaged whenthe engine is not running A variety of parking pawl designs are known inthe industry. Although the parking pawl is typically located within thetransmission case, other vehicle locations are possible. The parkingpawl may be mechanically linked to the range selector. In otherembodiments, controller 46 may control the engagement and disengagementof parking pawl 48 in response to movement of the range selector.

FIG. 2 illustrates how the shift elements of a transmission such as thetransmission illustrated in FIG. 1 are controlled to improve transitionsfrom Park to Drive, Reverse, or Neutral. At shown at 50, the vehicle isinitially in a condition in which the engine is off, the shift selectoris in the Park position, and the vehicle is held stationary by parkingpawl 48. At 52, when the engine is started, the controller engagesclutch 34 and brakes 38 and 40. This places gear box 24 in a tie-upstate as opposed to a neutral state. In other words, both turbine shaft20 and output element 16 are held against rotation. At 54, when thedriver moves the range selector to a non-Park position, such as Drive,Reverse, or Neutral, parking pawl 48 is disengaged. Because the shiftelements continue to hold the output shaft stationary, the vehicle doesnot move forward or backwards when the parking pawls is released and thereaction torque at the transmission mounts does not change. To avoidunintentional transitions from Park, the vehicle may have an interlockmechanism that prevents moving the shifter from Park when the brakepedal is not depressed.

The controller then waits at 56 until the brake pedal is released. Whenthe brake pedal is released, the controller releases one or more shiftelements depending on the state of the shift selector. For example, ifthe shift selector is in the Reverse position at 58, the controllerreleases brake 38 at 60 to place the transmission in reverse. The delaybetween commanding a disengagement and the torque capacity falling tonear zero is much shorter than the time delay associated withengagement. Therefore, the transmission reaches the reverse state muchsooner than it would if it were transitioning from a neutral state.Similarly, if the shift selector is in the Neutral position at 62, thecontroller releases two shift elements at 64 to place the transmissionin neutral. Finally, if the shift selector is in the Drive or Lowposition, the controller disengages clutch 34, placing the gear box in1st gear. In some embodiments, the controller may release clutches totransition to the selected transmission state without waiting forrelease of the brake pedal. When the vehicle stops with the brakesapplied, as determined at 68 and 70, the controller engages additionalclutches at 72 and returns to the waiting state 56. In this way,transitions between Drive and Reverse are also improved by going throughan intermediate tie-up state.

As shift elements 34, 38, and 40 are engaged, the gearbox may transitionbriefly through a reverse state or a 1st gear state before reaching thetie-up state. For example, if 34 and 40 are applied while 38 is stillbeing stroked, then the gear box would be in reverse until 38 isengaged. This can be undesirable if the driver moves the shift lever toDrive during this interval because the vehicle would move backward(though only briefly) when the driver is expecting to move forward. Toavoid this possibility, the controller may stage the engagement of theshift elements systematically to avoid transitioning through a reverseor forward gear state. The sequence will depend on the kinematicarrangement and the clutch application pattern. The process for gear box24 is illustrated in FIG. 3. Engaging brakes 36 and 40 at 76 holds theoutput against rotation although the turbine shaft is still free torotate. Once 36 and 40 are engaged, engaging clutch 34 and brake 38 at78 holds the turbine shaft against rotation. Finally, brake 36 isreleased at 80.

FIG. 4 illustrates a different gear box arrangement. Gear box 90establishes nine forward speed ratios and one reverse speed ratiobetween turbine shaft 20 and output element 16. Simple planetary gearsets 92, 94, 96, and 98 each have a sun gear, a carrier, and a ring gearthat rotate about a common axis. Each planetary gear set also includes anumber of planet gears that rotate with respect to the carrier and meshwith both the sun gear and the ring gear. The carriers of gear sets 92and 94 are fixedly coupled to one another and form intermediate shaft100. The sun gear of gear set 94 is fixedly coupled to the ring gear ofgear set 92. Clutches 102 and 104 selectively couple turbine shaft 20 tothe ring gear and sun gear, respectively, of gear set 92. Brakes 106 and108 selectively hold sun gear of gear set 92 and the ring gear of gearset 94, respectively, against rotation. Engaging these four shiftelements in combinations of two establishes a variety of speed ratiosbetween turbine shaft 20 and intermediate shaft 100. Engaging clutch 104and brake 108 cause intermediate shaft 1000 to rotate in the oppositedirection of turbine shaft 20. Engaging brakes 106 and 108 holdintermediate shaft 100 stationary. Engaging clutch 102 with brake 106 orbrake 108 cause intermediate shaft 100 to rotate slower than turbineshaft 20 at two different speed ratios. Finally, engaging clutches 102and 104 cause intermediate shaft 100 to rotate at the same speed asturbine shaft 20.

Gear set 98 and clutch 110 selectively establish an underdrive speedrelationship between intermediate shaft 100 and output element 16. Thus,engaging clutch 110 together with various combinations of two of shiftelements 102-108 establish the reverse ratio and the three lowestforward ratios. Gear set 96 and clutch 112 selectively establish alinear speed relationship among intermediate shaft 100, turbine shaft20, and output element 16. Thus, engaging clutch 112 together withvarious combinations of two of shift elements 102-108 establishes adirect drive ratio and four overdrive ratios. The remaining ratio isestablished by engaging clutches 110 and 112 together. Table 2illustrates which shift elements are engaged to establish each speedratio.

TABLE 2 102 104 106 108 110 112 Ratio Step Reverse X X X −3.09 69% 1st XX X 4.47 2nd X X X 2.66 1.68 3rd X X X 1.68 1.58 4th (X) X X 1.23 1.365th X X X 1.00 1.23 6th X X X 0.84 1.19 7th X X X 0.76 1.11 8th X X X0.66 1.15 9th X X X 0.56 1.19

When the engine is started, the controller engages clutches 102 and 104and brake 108 holding intermediate shaft 100 and turbine shaft 20against rotation. Then, the controller engages clutch 110 which holdsoutput element 16 stationary. This places gear box 90 in a tie-up state.When the driver moves the range selector to the Reverse position andreleases the brake pedal, parking pawl 48 is disengaged and thecontroller releases clutch 102 to place gear box 90 in reverse.Similarly, when the driver moves the range selector to the Driveposition and releases the brake pedal, the parking pawl is released andthe controller disengages clutch 104, placing the gear box in 1st gear.Finally, if the driver moves the range selector to Neutral and releasethe brake pedal, the parking pawl is released and the controllerreleases clutch 110, placing the gear box in a neutral state.

The processes, methods, or algorithms disclosed herein can bedeliverable to/implemented by a processing device, controller, orcomputer, which can include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, or algorithms can be stored as data and instructions executableby a controller or computer in many forms including, but not limited to,information permanently stored on non-writable storage media such as ROMdevices and information alterably stored on writeable storage media suchas floppy disks, magnetic tapes, CDs, RAM devices, and other magneticand optical media. The processes, methods, or algorithms can also beimplemented in a software executable object. Alternatively, theprocesses, methods, or algorithms can be embodied in whole or in partusing suitable hardware components, such as Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs),state machines, controllers or other hardware components or devices, ora combination of hardware, software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A method of operating a vehicle transmissioncomprising: while a park mode is selected and a parking pawl is engaged,engaging sufficient shift elements to hold a transmission output shaftand a turbine shaft against rotation with the parking pawl disengaged;and transitioning from the park mode to a non-park mode by disengagingthe parking pawl and disengaging one of the shift elements.
 2. Themethod of claim 1 wherein the non-park mode is a forward drive mode. 3.The method of claim 1 wherein the non-park mode is a reverse drive mode.4. The method of claim 1 wherein the non-park mode is a neutral mode. 5.The method of claim 1 wherein the parking pawl is disengaged before theshift element is disengaged to transition to the non-park mode.
 6. Themethod of claim 5 further comprising waiting for a brake pedal to bereleased before disengaging the shift element.
 7. The method of claim 1wherein the shift element engagements are sequenced to avoid temporarilyestablishing a power flow path from the turbine shaft to the outputshaft.
 8. A transmission comprising: a first shift element which isengaged in a park state and a forward launch state and disengaged in areverse launch state; and a second shift element which is engaged thepark state and the reverse launch state and disengaged in the forwardlaunch state.
 9. The transmission of claim 8 further comprising acontroller programmed to transition from the park state to the forwardlaunch state by disengaging the second shift element.
 10. Thetransmission of claim 8 further comprising a controller programmed totransition from the park state to the reverse launch state bydisengaging the first shift element.
 11. The transmission of claim 8further comprising a third shift element which is engaged in the parkstate, the forward launch state, and the reverse launch state.
 12. Thetransmission of claim 11 wherein the first and second shift elements areengaged and the third shift element is disengaged in a neutral state.13. The transmission of claim 11 further comprising a turbine shaft andan output shaft wherein the first, second, and third shift elementscollectively hold both the turbine shaft and the output shaft againstrotation in the park state.
 14. The transmission of claim 13 furthercomprising a torque converter having an impeller fixed to an input shaftand a turbine fixed to the turbine shaft.
 15. The transmission of claim11 further comprising a fourth shift element which is engaged in thepark state, the forward launch state, and the reverse launch state. 16.The transmission of claim 15 further comprising a turbine shaft and anoutput shaft wherein the first, second, third, and fourth shift elementscollectively hold both the turbine shaft and the output shaft againstrotation in the park state.